The exhaust box 1 of the LP module 2 and the envelope 3 of the condenser form a vast vacuum enclosure 4 that is made of sheet metal and that is reinforced by internal ties (see FIGS. 1 and 2).
The exhaust box 1 and the envelope 3 have their own specific functions:
The exhaust box 1 and the support-block 5 on which it rests support the active portion of the turbine, i.e. the rotor 6 and the LP body 7. It is the permanent or variable deformations of this assembly that govern the maintenance of radial clearances between the rotor and the LP body, and thus the proper dynamic behavior of the shaft line. The exhaust box 1 directs the steam towards the heat-exchange bundles 8 of the condenser 3.
The condenser envelope 3 stands on the raft 9 and only has to support the static load internal to the condenser 3, and in particular the condensation bundles 8.
The exhaust box 1 and the condenser envelope 3 are connected together to form said single vacuum enclosure 4.
If the connection 12 is rigid (e.g. by welding) then the enclosure 4 withstands force due to outside pressure, but it exerts varying forces on the support legs 10 of the exhaust box 1 and also on the raft (see FIG. 1); these varying forces are due to the varying mass of water contained in the condenser and to thermal expansion of the condenser. Thus, when the condenser cools, the load on the springs 11 diminishes while the load on the support legs increases.
If the connection 12 is elastic (FIG. 2) then the two enclosures are "mechanically" separate and forces due to outside pressure together with the weight of the LP module are supported by the support-block 5 through the support legs 10 of the exhaust box. The total mass of the condenser envelope 3 together with the forces due to the vacuum are supported by the foundation raft 9 without interposed springs 11.
Neither of the above two configurations is satisfactory since there is always interference and incompatibility between the turbine/support-block function and the condensation function.
All projects for making the condenser envelope out of concrete have been based on the above two principles that are practically unavoidable when the condenser is situated beneath the low pressure turbine. Thus, the block supporting the turbine and the concrete condenser envelope are intimately linked since the infrastructure of one is used for making the other.
In such a structure, there is therefore interference between the turbine/support-block function and the condensation function.
The drawbacks described above applicable to the use of a steel condenser envelope are thus still present, and in addition there are the displacements of the support-block and condenser structure due to the varying internal temperatures in the condenser which are detrimental to proper operation of the turbine.